Interpretive Summary: Root-knot nematodes are soil-born pests that attack and disfigure carrot roots, and only a small amount of nematode attack makes them unmarketable. In this study we report a strain of Asian carrots with genetic resistance to most root-knot nematodes, and we describe the nature of inheritance of this trait. This is the second source of genetic resistance to these pests, and this new discovery opens up the possibility of combining two genetic resistances which could potentially confer a stronger level of resistance. We call this new source of resistance Mj-2 and demonstrate a single gene accounts for part of the resistance. This research is of interest to carrot researchers, breeders, and growers since it demonstrates a scientifically interesting interaction between carrots and naturally occurring pests, and it provides a potential basis for reducing the need for pesticide application for successful carrot production.

Technical Abstract:
Root-knot nematodes limit carrot production around the world by inducing taproot forking and galling deformities that render carrots unmarketable. In warmer climates, Meloidogyne javanica and M. incognita are most prevalent. In F2 and F3 progeny from the cross between an Asian carrot resistant to M. javanica, PI 652188, and a susceptible carrot, resistance response was incompletely dominant with a relatively high heritability (h2 = 0.78) and evidence for a single gene, designated Mj-2, contributing to resistance. Molecular markers linked to the previously described root-knot nematode resistance gene, Mj-1 derived from ‘Brasilia’, demonstrated that Mj-2 does not map to that same locus, but may be on the same linkage group as Mj-1.